High society: behavioral patterns as a feedback loop to social structure in Plains bison (Bison bison bison)

Abstract

Social organization as a topic has wide application often conserved across lineages and can lead to complex cultures, yet it is still not well understood in many taxa. We observed American bison (Bison bison) to investigate the interactions of hierarchy and behavior to elucidate patterns of social organization. Bison are highly visible animals that live in relatively accessible grasslands, and most are in semi-wild conservation herds that allows for access to low-stress observation and physiological exams. We observed behaviors in a semi-wild, reintroduced herd of 53 bison in a 140-ha prairie from April to October 2015 (165 h of observation). After establishing a linear hierarchy, we categorized individuals of each sex as high, mid-, or low ranking and then evaluated behavioral trends across rank and sex. We found that high ranking males constantly defended their linear positions and focused breeding efforts on the females with the highest productivity, consistent with disruptive sex characteristics. Intra-rank conflict focused on linear positioning likely causes the volatility in male bison hierarchy and stress of domination. Female bison, however, tend to suppress the lowest class, which likely drives their hierarchy's stability and stress of subordination. Our results show that male and female bison are impacted differently by their social rankings and show that individual aggressive actions may provide feedback loops to their social organization.

Not Singing in the Rain: Linking Migratory Songbird Declines With Increasing Precipitation and Brood Parasitism Vulnerability

“…The Crane Trust, a non-profit organization, now maintains these grasslands through managed disturbances including grazing, haying, and burning, which control woody encroachment. Cattle, brought in seasonally, were the sole grazers until 2015, when a semi-domesticated bison herd was introduced year-round to part of our study area (King et al, 2019). Cattle and bison grazing often overlap with the songbird breeding season, while haying and burning typically take place during the non-breeding season, in the fall and spring, respectively.…”

Trade-offs between fighting and breeding: a social network analysis of bison male interactions

“…Studies of large, naturalistic herds of wild bison have reported that the dominance hierarchy of males is nonlinear, transitory, and unstable over the course of the breeding season ( Lott 1979 ), with frequent challenges and many rank reversals. King et al (2019) described similar “frequently shifting” dominance structures in a smaller, semiwild reintroduced herd. During the hectic 6-week breeding peak, male competitive ability is constantly changing, and the most successful males must accurately judge which rival males they can displace to gain copulations ( Wyman et al 2008 , 2012 ).…”

“…In support of the notion that dominant males invest in matings that are more likely to result in offspring compared with subordinate males, a study of semiwild bison in Nebraska found that high-ranking males focused breeding efforts on the females with the highest productivity ( King et al 2019 ). In fact, the top two dominant males focused most of their breeding efforts on the 11 females that produced 71% of the calves ( King et al 2019 ). Mooring and Penedo (2014) found that male dominance rank, but not observed mating events, was a significant predictor of paternity for a given calf.…”

“…During the hectic 6-week breeding peak, male competitive ability is constantly changing, and the most successful males must accurately judge which rival males they can displace to gain copulations ( Wyman et al 2008 , 2012 ). These vigorous breeding activities eventually result in elevation of glucocorticoid stress hormones, exhaustion, and subsequent rank loss by dominant males ( Mooring et al 2006b ; King et al 2019 ). Male physical condition deteriorates as the rut progresses, with males bellowing less frequently ( Berger and Cunningham 1991 ) and at lower amplitudes ( Wyman et al 2008 ) due to fatigue and spending less time foraging or resting ( Mooring et al 2006a ).…”

“…Although some breeding takes place into late fall and early winter, most females have bred by the end of August. The peak rut is short-lived, high-stakes, physically demanding, and chaotic—it is no wonder that male dominance structure at this time has been described as “tumultuous” and “volatile” ( King et al 2019 ).…”